What mechanics allow these birds to swarm without crashing into each other?

Starling flocks have no leader. Each bird in the flock obeys a few simple rules: Stick with your own kind, maintain your “personal space” envelope (tight to each side, but roomier in front), and follow the direction of the flock. Computer simulations have demonstrated that these basic rules are sufficient to create a virtual flock that behaves very much like a starling flock.

The way turns are initiated and propagated in the flock is simple too. In starling flocks, each individual bird monitors the motion of the seven nearest birds in the flock. This means that it doesn’t have to wait until the bird next door moves to know that the change is coming. It’s similar to the way a “wave” moves through the crowd at a football stadium. Only birds have much faster reaction times than humans, so the turn can propagate through the whole flock in milliseconds. Any one bird in the flock can initiate a change in direction, and the whole flock will respond just by following these rules.

Fantastic video! Yep, @thorninmud nailed it. I often get to watch similar, but smaller flock formations of birds across my pastures here.

@thorninmud Starling flocks have no leader.
We can’t say those birds were startled as there is no evidence in the clip to show that. At first, they were calmly resting on the wires and some were even in what look to be calm flight.

Each bird in the flock obeys a few simple rules: Stick with your own kind, maintain your “personal space” envelope (tight to each side, but roomier in front), and follow the direction of the flock.
That is established by whom? I guess some birds like these gulls, missed the memo because even though they swarm, which I have seen a massive swarm larger than those a few times around town, even having gulls travel into the swarm from seemingly half a mile away, they are not locked in tight formation and seem to have more of a maelstrom going; but still no crashes. Certainly the gulls I seen was not startled.

The way turns are initiated and propagated in the flock is simple too. In starling flocks, each individual bird monitors the motion of the seven nearest birds in the flock.
That may work for that species of bird but that has no explanation for the gulls who are not turning in the same direction or caring about ”personal space”, if there is any monitoring going on, it is to avoid smacking into another gull, not moving in unison with them.

@Coloma @thorninmud nailed it
But the stud got missed and the nail is hanging in drywall with no anchor.

@Hypocrisy_Central “We can’t say those birds were startled…”
Not “startled”, starlings. That’s the kind of bird in your video. It’s the species most frequently studied by scientists interested in flocking behaviors because they fly in the tightly coordinated formations shown in this video.

Other birds have other flocking behaviors and flock for different reasons. Starlings flock together as a defense mechanism against predators, geese flock because it gives them an aerodynamic edge in their long migrations, and seagulls mostly just go where they see other gulls gathering because that’s probably where the food is. But you posted a video of starlings and asked about how they flock. My answer is based on what current research has shown about starlings, not gulls.

@thorninmud It’s the species most frequently studied by scientists interested in flocking behaviors because they fly in the tightly coordinated formations shown in this video..
So, scientist let loose falcons, or hawks to determine this behavior was for defense? And how did they test that their ability to change direction as a unit, so-to-speak, was because they were so fast at direction change?

Here’s one recent study: Propagating waves in starling, Sturnus vulgaris, flocks under predation (Procaccini et al). You probably can’t access the full article, but what they did was videotape and analyze large flocks of starlings at two urban winter roosting sites in Italy over two years. There was no need to release falcons, because hawks and falcons regularly prey on the flocks. In total, 81 episodes of flocks under attack by falcons were recorded in HD and analyzed in slow motion.

The data are very detailed and complex, but here’s a paragraph summarizing their findings:

“Wave events (a train of several observable pulses of optical intensity that propagates along a given direction across the flock) are a straightforward example of emerging complexity: a simple functional response (escaping from predators), when affecting even just a relatively small portion of a cohesive flock of thousands of individuals, can produce very complex patterns. Probably, in birds, wave events occur only when the perturbation is so strong that individuals almost come into contact, which is probably the reason why they form only in the presence of a threat. Wave events possess a measurable antipredator function: it is likely that rapid variations, in local density and in both predator–prey and prey–prey distances, perturb the visual perception of falcons. As suggested for similar phenomena (e.g. Trafalgar wave) in giant honeybees (Kastberger et al., 2008 and Schmelzer and Kastberger, 2009), dunlins (Buchanan et al. 1988) and marine insects (Treherne & Foster 1981), we found that, also in starlings, waves can hamper predation success.”